High purity gases such as nitrogen in which impurities are present in amounts well below part per million levels are required in the manufacture of integrated circuits to prevent defects in chips of increasing line densities. Cryogenic distillation is typically used for the production of highly purified nitrogen gas.
Removal of impurities from the feed gas for cryogenic distillation is required for the production of high purity nitrogen. When air is used as the feed gas, impurities, such as H.sub.2 O and CO.sub.2, have to be removed to prevent freeze-up in the low temperature sections of the plant while other impurities, such as H.sub.2 and CO, have to be removed to prevent contamination of the nitrogen product.
A two-step procedure has been employed for the removal of these impurities from air in a nitrogen production process. In the first step, a compressed feed gas is heated to temperatures between 150.degree. to 250.degree. C. and then contacted with a catalyst to oxidize CO to CO.sub.2 and H.sub.2 to H.sub.2 O. Noble metal catalysts, typically based on platinum, are commonly used for the oxidation step. In the second step, the oxidization products, CO.sub.2 and H.sub.2 O, are removed from the compressed gas stream either by a temperature-swing adsorption process (see K. B. Wilson, A. R. Smith and A. Theobald, IOMA BROADCASTER, Jan-Feb 1984, pp 15-20) or by a pressure-swing adsorption process (see M. Tomonura, S. Nogita, Kagaku Kogaku Ronbunshu, Vol. 13, No. 5, 1987, pp 548-553).
These processes, although effective, are disadvantageous for the commercial scale production of highly purified gases, particularly nitrogen gas due to their high cost of operation. The cost of operation is high because of the extensive use of expensive noble metal catalysts. In addition, separate vessels must be used for the catalytic treatment step and the adsorption step to remove the impurities. In addition, heat exchangers are required to both heat the gas as it passes into the catalyst vessel and cool the effluent therefrom. This poses additional costs, both in terms of equipment and energy.
Low temperature processes for removing parts per million levels of impurities from inert gas streams are also known in the art. Weltmer et al., U.S. Pat. No. 4,579,723, discloses passing an inert gas stream containing nitrogen or argon through a catalytic bed containing a mixture of chromium and platinum on gamma-alumina and a second bed composed of a mixture of several metals mounted on gamma-alumina. The beds effectively convert carbon monoxide to carbon dioxide and hydrogen to water vapor and adsorb the resulting impurities to produce a product stream containing total impurities of less than 1.0 part per million.
Tamhankar et al., U.S. Pat. No. 4,713.224, discloses a one-step process for purifying gases containing minute quantities of CO, CO.sub.2, O.sub.2, H.sub.2 and H.sub.2 O in which the gas stream is passed through a particulate material comprised of nickel having a large surface area.
Processes for the ambient temperature conversion of carbon monoxide to carbon dioxide have also been described as, for example, by Tamura et al., U.S. Pat. No. 3,672,824, and Frevel et al., U.S. Pat. No. 3,758,666.
None of these processes, however, provide an integrated low temperature system in which a feed stream which contains up to significant amounts of impurities can be treated in an efficient and inexpensive manner to obtain highly purified gaseous products which can be subsequently treated to produce high purity gases, such as nitrogen.
It is, therefore, an object of the present invention to provide a process for producing highly purified gaseous products from a feed stream contain- ing up to significant amounts of impurities.
It is another object of the invention to provide a process for purifying oxygen-containing gas streams suitable for the production of highly purified nitrogen by cryogenic distillation.